1. Field of the Invention
The present invention relates generally to fluid flow control and, more particularly, to high resolution flow control of, for example, high pressure compressible fluids including supercritical fluids.
2. State of the Art
Control of fluid flow is important in numerous applications. For example, fluid flow control is involved in hydraulic applications, in the operation of various semiconductor fabrication systems such as chemical vapor deposition (CVD) and atomic layer deposition (ALD) equipment, in the operation of autoclaves and similar equipment, and in the performance of various laboratory experiments.
In all of the above-listed applications, as well as numerous others, the ability to control fluid flow, whether according to pressure or flow rate (either mass or volume), is important to the success of the operation or process being performed. For example, in regard to various laboratory experiments, fluid flow control often needs to be precise and repeatable so as to ensure that certain input conditions are actually what was intended and the integrity of the experiment's outcome is not in question. It becomes even more important to control the fluid flow, and also more difficult to accurately do so, when the fluid being handled is supercritical and there is a potential of effecting a phase change within the fluid as it flows through a flow control device. While various fluid control devices have been designed in an attempt to provide high resolution flow control, such devices have been lacking in their ability to consistently provide accurate control of fluids including high pressure, compressible fluids.
For example, referring to FIG. 1, a prior art flow control device 10 is shown. The flow control device 10 includes a valve 12 with a flow path 14 defined therethrough. The valve includes an inlet 16 configured to be coupled with a fluid source (not shown) and an outlet 18 configured to be coupled with a conduit or some other device to which fluid is to be delivered (none shown). A linearly positionable valve stem 20 is disposed within the valve and configured to control the flow of fluid passing through the defined flow path 14. Packing 22 or some other seal arrangement may be disposed about a portion of the valve stem 20 to prevent leaking of the fluid around the valve stem 20. The valve stem 20 is coupled with a linear positioning actuator 24 which displaces the valve stem along a linear path as indicated by directional arrow 26.
While the flow control device 10 may provide adequate fluid flow control for some applications, it is desirable to improve on such an arrangement. For example, a flow control device configured substantially as described with respect to FIG. 1 may exhibit a flow coefficient of approximately 0.03 Cv, wherein Cv may be defined, as it relates to valves, as a quantity relating a flow rate, in gallons per minute (gpm), of a fluid with a known specific gravity to the pressure drop experience across the valve as measured in pounds per square inch (psi). It may be noted that the flow coefficient is not dimensionally homogenous (as illustrated in the following equations) and is specifically limited to English units.
For incompressible fluids the flow coefficient Cv may be expressed by the following equation:
      C    v    =      Q                            Δ          ⁢                                          ⁢          p                          S          ⁢                                          ⁢          G                    
Wherein Q is the flow rate in gallons per minute, Δp is the change in pressure across the valve in pounds per square inch, and SG is the specific gravity of the fluid flowing through the valve.
For compressible fluids, the determination of the flow coefficient becomes more complex. For example, if the inlet pressure is twice that of the outlet pressure (what may be termed as critical flow) or greater, the flow coefficient may be expressed by the following equation:
      C    v    =            Q      G        ⁢                            S          ⁢                                          ⁢          G          ×          T                            816        ×                  P          inlet                    
If the inlet pressure is less than twice the outlet pressure (what may be termed subcritical flow) the flow coefficient may be expressed by the following equation:
      C    v    =                    Q        G            962        ⁢                            S          ⁢                                          ⁢          G          ×          T                                      P            inlet            2                    -                      P            outlet            2                              
Wherein QG is the flow rate of the fluid in standard cubic feet per minute (scfm), T is the absolute temperature in degrees Rankin, Pinlet and Poutlet are the inlet and outlet pressures of the valve, respectively, in pounds per square inch absolute (psia), and SG is the specific gravity of the fluid flowing through the valve.
Returning to the prior art flow control device 10 described with respect to FIG. 1, while in absolute terms, a flow coefficient of 0.03 Cv would appear to provide fluid control at what might be consider a “high” resolution, such a flow coefficient may not be considered adequate for a number of applications including. For example, in some applications, such as various laboratory experiments, it may be desired to provide flow control with a resolution which is approximately an order of magnitude finer than such a prior art flow control device. Additionally, such a flow control device 10 has, in the past, only provided adequate pressure control of a fluid within, for example, 50 to 100 psi in some cases. It is desirable to obtain more exact pressure control of the fluid for numerous applications.
An additional problem with the flow control device 10 shown and described with respect to FIG. 1 is that the linear motion of the valve stem 20 makes the valve 12 vulnerable to contamination from grit or small particulates which may be present in the fluid flowing therethrough. For example, in the past, such a valve 12 has had small particulates become lodged or wedged between the valve stem 20 and the valve stem seat 28. When lodged between the valve stem 20 and valve stem seat 28, the particulates have interfered with the actuation of the valve stem 20 and the precise positioning thereof. Furthermore, the presence of particulates between the valve stem 20 and the valve stem seat 28 has, in the past, resulted in the galling of the two components thereby causing the valve 12, initially, to operate imprecisely and, ultimately, to fail. In some particular cases, the valve 12 associated with a flow control device such as described with respect to FIG. 1 has failed within approximately fifteen to twenty minutes of use because of the presence of such particulates in the fluid.
In view of the shortcomings in the art, it would be advantageous to provide a method and apparatus for consistently and repeatedly controlling the flow of high pressure, compressible fluids at a relatively high resolution. It would further be desirable to provide a method and apparatus of controlling fluid flow which is not susceptible to fouling or galling due to the presence of particulates within a fluid being processed thereby.